Apple’s exhorbitantly-priced Mac storage comes with impressive benchmark claims but has some serious, hidden bottlenecks in certain scenarios, as do other SSDs. David Harry illustrated this clearly in a video, using Activity Monitor as a novel window into dramatic SSD slowdowns when moving large amounts of data to an Apple internal SSD from a third-party Thunderbolt 5 SSD.
He actually finds that the external SSD, connected via Thunderbolt 4, is faster at reading than the MacBook Pro’s internal one. Write performance drops once the SLC cache is full.
In normal use, there are three potential causes of reduced write speed in an otherwise healthy SSD:
- thermal throttling,
- SLC write cache depletion,
- the need for Trimming and/or housekeeping.
[…]
To achieve their high storage density, almost all consumer-grade SSDs store multiple bits in each of their memory cells, and most recent products store three in Triple-Level Cell or TLC. Writing all three bits to a single cell takes longer than it would to write them to separate cells, so most TLC SSDs compensate by using caches. Almost all feature a smaller static cache of up to 16 GB, used when writing small amounts of data, and a more substantial dynamic cache borrowed from main storage cells by writing single bits to them as if they were SLC (single-level cell) rather than TLC.
[…]
Keep ample free space on the SSD so the whole of its SLC write cache can be used.
The price on added memory and SSD capacity on the otherwise excellent Mac Mini is horrible. I wanted to avoid paying the apple tax on that storage, so I am using an external SSD and it means I can get twice the storage for less than half the price. But I wanted to find out what it would mean for performance, and I couldn’t believe what I found.
The Mac mini also has Thunderbolt 4, but he finds it limiting performance. The internal 256 SSD seems to be slow, despite what was reported earlier about it using multiple chips like the larger versions.
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